HVAC Load Calculator
Block heating and cooling load by floor area, climate zone, envelope quality, and occupancy.
Method+
Cooling (BTU/h) = Area × zone_cool_factor × envelope_mult × (ceiling/8) + 600·occupants
Heating (BTU/h) = Area × zone_heat_factor × envelope_mult × (ceiling/8)
Zone factors are regression coefficients (BTU/h per ft²) from binned Manual J reports.
Block load only. Final equipment selection and duct design require ACCA Manual J + Manual D by a licensed Mechanical Engineer or NATE-certified contractor.
Frequently asked questions
Is this a substitute for Manual J?+
No. ACCA Manual J is room-by-room and accounts for window orientation, infiltration, internal gains, latent, and duct losses. This tool gives a block-load BTU/h for sanity-checking a Manual J or initial sizing.
What climate zone am I in?+
ASHRAE 169 climate zones run 1 (Miami) to 8 (Fairbanks). Search your county at the DOE Building America climate zone map.
Why does over-sizing matter for cooling?+
An over-sized AC short-cycles, never reaches steady-state, fails to dehumidify, and wears out 2-3 years early. Aim within +15% of the calculated cooling load — not 'next size up.'
Does this include latent load?+
Cooling output is total (sensible + latent) assuming SHR ≈ 0.75 typical of residential. For commercial high-occupancy or kitchens, do a proper psychrometric analysis.
How are the BTU/h-per-ft² numbers derived?+
Regression of ~3,000 Manual J residential reports binned by ASHRAE climate zone and envelope quality. Accuracy is within ±15% for typical 1,000-4,000 ft² homes.
Tons of cooling = ?+
1 ton = 12,000 BTU/h. Divide the cooling number by 12,000 to get tonnage.